Dependence of 50‐keV upstream ion events at IMP 7&8 upon magnetic field bow shock geometry

We present the results of a statistical study of 4 years (1972–1976) of IMP 7 and 8 observations at ∼40 R E of 50‐ to 200‐keV upstream ion events measured with the JHU/APL Energetic Particle Experiment (EPE). We find a monotonic increase in the probability of observing upstream particle events with a decrease in the angle ( θ Bn ) between the interplanetary magnetic field (IMF) and the local shock normal at the point where the IMF intersects the bow shock, independent of the length of time of bow shock connection (if it exceeds ∼10 min). We find roughly equal probability of observing an event above a given flux from any portion of the bow shock with the same value of θ Bn , a growth time of the 50‐ to 200‐keV events of ∼10 min, a maximum attainable flux of ∼2.5×10 4 (cm²s sr) −1 , and a positive correlation between the probability of exceeding a given flux and the 3‐hour K p index. The results imply that the local structure of the bow shock in the immediate vicinity of the field line connection is the dominant influence in the generation process of energetic upstream particle events and that wave‐particle interactions produce a self‐throttling mechanism that limits the maximum flux of ions escaping the upstream foreshock.

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